Signal analysis in leaky lamb wave nde technique

ABSTRACT

A system and method for detecting defects in composite laminate material are described which comprise a first trtansducer for directing an ultrasonic beam of preselected frequency and tone burst excitation along a transmission axis through a sonic coupling bath onto a surface of the material at a preselected angle to generate Lamb waves in the material and leaky Lamb waves reflected therefrom, a frequency modulator for selectively modulating the frequency of the incident beam, and a second transducer for receiving the reflected sonic field including leaky Lamb waves reflected from the material and for providing a recovered and envelope-detected output signal corresponding to the amplitude of the reflected waves.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

This application is a continuation-in-part of application Ser. No.18,382, filed Feb. 24, 1987, now abandoned.

CROSS REFERENCE TO RELATED APPLICATION

The invention described herein is related to U.S. Pat. No. 4,674,334,dated June 23, 1987, granted on copending application Ser. No. 865,507filed May 13, 1986, and entitled "Properties of Composite LaminatesUsing Leaky Lamb Waves".

BACKGROUND OF THE INVENTION

The present invention relates generally to nondestructive testingsystems and methods using ultrasonics, and more particularly to improvedsystem and method for nondestructive testing of composite laminates.

Composite materials, particularly fibrous composite laminates are ofsubstantial interest to the aerospace industry for aircraft structuralcomponents by reason of high strength-to-weight ratios whichcharacterize these materials. In the manufacture and use of compositematerials, any of numerous performance limiting defects may arise withina composite structure, including delaminations, porosity, voids, plygaps and overlaps, resin-rich and resin-lean regions, fibermisalignment, and cracks. Certain defects not detected in the laminatestructure as manufactured may arise in the material during use.Nondestructive testing (NDT) methods in use heretofore for detection ofthe defects include x-ray radiography and ultrasonics in various modessuch as pulse-echo and through-transmission. However, the inhomogeneousanisotropic structure which characterizes fibrous composites severelylimits the suitability of conventional NDT methods for these materials.

According to the teachings of the cross reference, leaky Lamb waves maybe used to detect and characterize defects in fibrous compositelaminates, which defects may not be detectable using conventional NDTtechniques. Leaky Lamb waves are excited in water-coupled compositelaminate plates by ultrasonic isonification at appropriate angles andfrequencies using a two transducer, pitch-catch geometry. The resultingleaky Lamb wave modes have minima which are characteristic of defectsextant in the laminate including plate thickness, fiber volume fractionand resin content, variations of which within a few percent may not besignificant or objectionable. The teachings and background materialdisclosed in the cross reference are incorporated here by reference.

The invention substantially solves certain shortcomings of existing NDTtechniques and is a significant improvement over the teachings of thecross reference with respect to defect discrimination particularly indetection of delaminations and porosity in laminates with small localthickness variations. According to the invention, periodicity of leakyLamb wave spectra, instead of single mode minima, provides a basis forthe nondestructive evaluation of the laminate. The invention permitssuppression of signals caused by changes in leaky Lamb wave modestructure resulting from small variations in laminate thickness, fibervolume fraction or resin content, but retains sensitivity to defects ofinterest, such as delaminations and porosity. In the practice of theinvention, the frequency of a radio frequency (RF) tone burst ismodulated over a range of from about 50 to 150% of the transducer centerfrequency at a rate of about 2 to 20 Hz. A frequency modulated toneburst is a gated radio-frequency signal where the radio-frequency isessentially constant within each individual burst, but where theradio-frequency changes slowly from burst to burst in a stepwise andperiodic fashion. The modulation, after sonic interaction with thelaminate, results in a time-domain signal, which varies nearlyperiodically at the output of a gated integrating amplifier. This outputsignal may in turn be filtered or processed in other manner to extractspecific defect-related information, to detect sound material, and/or tosuppress effects on the signal of small variations in test articlethickness or fiber, volume fraction. Defects such as delaminations orporosity will change or disrupt the periodicity of mode excitationacross the modulation band in a way that the effective repetition rateof the envelope-detected video signal, after interaction with thelaminate, changes significantly in the presence of these flaws. Furthersignal processing, described below, allows the method to distinguishamong flaws of various type and location in the laminate layup, but toignore signal response from plate thickness changes at any preselectedlevel of suppression.

It is therefore a principal object of the invention to provide animproved nondestructive evaluation system and method.

It is a further object of the invention to provide system and method fornondestructively testing composite materials.

It is another object of the invention to provide nondestructive testingsystem and method utilizing leaky Lamb waves.

It is another object of the invention to provide nondestructive systemand method utilizing leaky Lamb waves and signal coding and decoding fordistinguishing defects from small thickness variations in laminatematerial.

These and other objects of this invention will become apparent as thedetailed description of representative embodiments proceeds.

SUMMARY OF THE INVENTION

In accordance with the foregoing principles and objects of theinvention, system and method for detecting defects in composite laminatematerial are described which comprise a first transducer for directingan ultrasonic beam of preselected frequency and tone burst excitationalong a transmission axis through a sonic coupling bath onto a surfaceof the material at a preselected angle to generate Lamb waves in thematerial and leaky Lamb waves reflected therefrom, a frequency modulatorfor selectively modulating the frequency of the incident beam, and asecond transducer for receiving the reflected sonic field includingleaky Lamb waves reflected from the material and for providing arecovered and envelope-detected output signal corresponding to theamplitude of the reflected waves.

DESCRIPTION OF THE DRAWINGS

The invention will be understood from the following description ofrepresentative embodiments thereof red in conjunction with theaccompanying drawings wherein:

FIG. 1 is a schematic of the system used in generation and detection ofleaky Lamb waves according to the cross reference;

FIG. 2 is a plot of receiver signal versus frequency for a swept toneburst reflected from graphite epoxy composite at 22°;

FIG. 3 presents representative experimental velocity dispersion curvesof frequency times thickness versus phase velocity for a typicalunidirectional graphite epoxy composite;

FIG. 4 is a block diagram of the system of the invention and useful inthe practice of the method thereof;

FIG. 5 is a leaky Lamb wave C-scan of 28×14 cm composite 4.5 mm thickwith embedded defects using no frequency modulation; and

FIG. 6 is a two-dimensional scan of the material of FIG. 5 using toneburst excitation frequency modulation, and audio-frequency filteringaccording to the invention.

DETAILED DESCRIPTION

Theoretical discussions on layered and fibrous composite models andexperimental measurements related hereto are given in "Leaky Lamb Wavesin Fibrous Composite Laminates", by D. E. Chimenti and A. H. Nayfeh, JAppl Phys 58:12, 4531-38 (Dec. 15, 1985) and "Anomalous UltrasonicDispersion in Fluid-Coupled Fibrous Composite Plates", by D. E. Chimentiand A. H. Nayfeh, Appl Phys Lett 49:9, 492-93 (Sept. 1, 1986), which areincorporated by reference herein.

Referring now to the drawings, shown in FIG. 1 is a schematic of asystem for leaky Lamb wave generation and detection for compositetesting described in the cross reference. Ultrasonic transducers 11,12are positioned over composite sample plate 13 within suitable soundcoupling fluid 14 (e.g. water) so that the respective principalultrasonic wave propagation axes 11a,12a define a plane perpendicular toplate 13 and intersect at a preselected point below the upper surface ofplate 13; transducers 11,12 are movable relative to each other along theupper surface of plate 13 in order to position axes 11a,12a atappropriate angles for generation and detection of leaky Lamb waves.Transducer 11 transmits ultrasonic beam 15 of selected frequency of fromabout 0.5 to about 15 MHz along axis 11a at angle A preselectedaccording to the specific material of plate 13 being examined; the rangeof angle A of from about 8° to 75° relative to a normal to plate 13 wasfound in the cross reference to be useful in the generation anddetection of leaky Lamb waves in plate 13. Specular reflection of beam15 from the surface of plate 13 is defined by dashed lines 15a. LeakyLamb wave modes are identified by adjusting axes 11a,12a of transducers11,12 to equal angles relative to vertical in a pitch-catchconfiguration. In the present invention, the transmitter transducer isdriven by a frequency modulated RF tone burst. While it would bepossible under restricted conditions to apply the method describedherein using a continuous wave (CW) signal source, the clearlypreferable embodiment of the invention exploits the advantages of toneburst excitation. These are: (1) the isolation of potentially disturbingechoes from surrounding structure or the fluid medium boundaries, (2)ready applicability of time-domain signal averaging techniques, (3)maintenance of quasi-CW excitation, permitting use of frequency-domainanalysis. A suitable tone burst is about 10 to 200 microsec in durationat RF values of 1 to 20 MHz with a burst on/off duty factor of about0.001 to 0.02. The reflected signal received by transducer 12 isacquired using a computerized area scanning system. Leaky Lamb wavesgenerated by interaction of beam 15 with plate 13 are represented by theshaded region and the arrows in FIG. 1 in the region bearing the legendLW. According to teachings of the cross reference, interaction of thereflected component of appropriate wavelength and beam width results ina phase cancellation, manifested as a characteristic amplitude minimumor null zone N, which occurs within a fairly narrow range of wavelengthand incidence angle. That is, the identification of Lamb modes isachieved by noting the occurrence of amplitude minima at particularincident angles and frequencies determined by the dispersioncharacteristics of the plate, viz. FIG. 3, as taught in the crossreference. Once leaky Lamb wave excitation is identified in a knowndefect-free plate, an appropriate scan is made at selected frequencywith the transducers oriented to observe null zone N. The signal levelat null zone N provides a sensitive measure of variations in thestructure of plate 13 resulting from local changes of properties or fromdiscontinuities and defects.

Referring now to FIG. 2, shown therein is a plot 21 of receiver signalversus frequency for a swept-frequency tone burst reflected from 0.92 mmthick graphite epoxy plate at a 22° incidence angle; leaky Lamb wavemodes correspond to the observed minima 23a,b,c,d and others at higherfrequencies in the reflected field signal above the frequency spectrumshown.

Referring now to FIG. 3, shown therein are experimental velocitydispersion curves 31a-f of frequency times thickness versus phasevelocity for a representative graphite epoxy composite. In accordancewith a governing principle of the invention, it is seen from the plotsof FIG. 3 that in the generation of leaky Lamb waves in this material,mode position is relatively insensitive to angle of incidence (i.e.,phase velocity) for phase velocities near 5 km/sec, and occurrence ofmodes (i.e., minima) between about 3.5 and 7 km/sec is relativelyperiodic, which forms the basis for a signal coding and processingmethod taught in the invention and which permits insignificant thicknessvariations in the composite to be distinguished from other defects ofinterest.

Other materials having Lamb wave dispersion characteristics differentfrom those illustrated in FIG. 3 may also benefit from the invention byapplication of the following general procedure for selecting a usefuloperating point. The angle of incidence should be preselected such thatthe wave phase velocity (equal to the quotient of the fluid velocity andthe sine of the incident angle) falls approximately halfway between thetransverse critical velocity and the longitudinal critical velocity inthe material examined. Further the range of frequency sweep should beselected to include several Lamb wave modes for optimum discriminationpotential. An approximate rule of thumb to make this selection is tocompute the ratio of transverse wavespeed to plate thickness. Thefrequency range should then be no less than twice this ratio and may becentered at a frequency chosen for convenient operation of conventionaltransducers and associated electronics.

Referring now to FIG. 4, shown therein is a block diagram of equipmentuseful in the practice of the method of the invention. Frequencymodulator 41 is operatively connected to RF signal generator 42, whichin turn passes the signal through RF gate 43, producing a frequencymodulated tone burst of preselected burst length. This signal may beincreased in amplifier 44, and it is then passed to transmittertransducer 45 having its sonic propagation axis 45a disposed withrespect to laminate workpiece sample 46 at appropriate angle, asdiscussed above, for the generation of leaky Lamb waves; the reflectedfield is detected by receiver transducer 47 having its axis 47a orientedto sample 46 as with transducer 45. As suggested in FIG. 4, transducers45,47 are further disposed relative to sample 46 such that axes 45a,47aintersect within sample 46 a preselected distance below the surfacethereof. The equipment arrangement displayed in that portion of FIG. 4including transducers 45,47 and workpiece sample 46 and including soundcoupling medium 48 (e.g. water) is equivalent to the arrangementdisplayed in FIG. 1. The signal from transducer 47 is strengthened byamplifier 49, which is operatively connected to video envelope-detector50, removing the RF content from the signal. The resultant RF envelopewaveform passes to a gated integrating amplifier 51, which recovers theamplitude information from the video-detected tone burst. A narrow-bandaudio-frequency (AF) bandpass filter 52, whose preselected centerfrequency (preferably in the range of about 10 to 200 hz) is determinedby the response function of the plate under test, allows selectivesuppression of unwanted signal information by passing only that signalinformation falling within a preselected bandwidth (about 1 to 50 Hz)corresponding to small variations in the rate of occurrence (dependent)on the thickness and elastic properties of the plate and the modulationfrequency) of the sample-induced amplitude minima at filter 52. Thefiltered time-varying signal is measured with an integrating ACvoltmeter 53 or equivalent signal processing means to provide an outputcorresponding to the observed defects in sample 46.

In the practice of the method of the invention, the frequency of the RFtone burst from transducer 45 is modulated over a range of about 50 to150% of the center frequency of transducer 45 at a rate of from about 2to about 20 Hz. The waveform with which the RF tone burst is frequencymodulated may be selected by the skilled artisan guided by theseteachings, a triangular waveform selected for demonstration of theinvention being a preferred selection. This modulation produces atime-domain signal received by transducer 47 and analyzed by thereceiver electronics which varies approximately periodically, dependingon the nature and location of the leaky Lamb wave modes in the reflectedfield. Small thickness variations in sample 46 result in aproportionately small shift in the effective repetition rate of thesignal received from transducer 47, whereas a delamination or porositydisrupts the mode excitation across the modulation band, and therepetition rate changes significantly. Selective bandpass filtering ofthe output signal and its detection by voltmeter 53 permits the systemto distinguish between thickness variations and the stated defects ofinterest. It is understood that alternate means of frequency analysis ofthe detected waveform, such as digital signal processing, may be used toconstruct a pseudo-frequency spectrum in the AF range, upon whichdigital filtering or other suitable operations on the frequency domaininformation characteristic of a sample under examination can beperformed to achieve the desired result.

The principle of the invention therefore is to exploit the relativeperiodicity of the generation of leaky Lamb waves especially incomposites, instead of the individual mode excitation itself, to makemanifest the presence of defects in the plate, while selectivelydiscriminating against small property or thickness variations which arejudged, before the fact, not to be significant. This method isaccomplished by suitably coding and processing the signals directed atand received from the plate immersed in a fluid. Study of the dispersioncharacteristics of plates, especially composite plates, reveals anappropriate choice for signal coding and processing, indicated above, toachieve this end. Frequency modulation of the RF tone burst whichexcites the leaky Lamb waves produces at the receiving transducer 47 anearly periodic, time-dependent signal. The information in this signalis utilized by first performing an envelope detection on the RF andsampling its level as a function of time, then bandpass filtering thesampled video. Alternatively, one sweep of the sampled video signal maybe frequency analyzed to yield an AF spectrum. Small property orthickness variations have a proportionately small influence on theperiodicity of the sampled video signal. A suitable choice for the passband width of the filter allows the method to ignore such variations,while retaining sensitivity to defects of interest.

Referring now to FIGS. 5 and 6, shown in FIG. 5 is a leaky Lamb waveC-scan 50 of 28×14 cm composite sample 4.5 mm thick with embeddeddefects; the scan was taken using a constant RF tone burst frequency (nofrequency modulation). FIG. 6 is a corresponding C-scan 60 of the samematerial of FIG. 5 taken according to the method of the invention using10 Hz tone burst modulation over 75% of the center frequency (2.2 Hz) ofthe incident beam. Transducers 45,47 of the system depicted in FIG. 4were scanned in tandem across the sample to obtain the data shown in therespective C-scans of FIGS. 5 and 6. The material of FIGS. 5,6 was typeAS4/3501-6 with (0)32 lay-up. Controlled defects in the sample includeddouble layers of Teflon® wafers 12.7 microns thick three such doublewafers being embedded between layers 8 and 9, 16 and 17, and 24 and 25.Porosity was simulated using glass microspheres with an average diameterof 40 microns and shell thickness of about 2 microns embedded betweenlayers of the composite sample. Resin rich regions were simulated by 6.4mm cuts through two successive plies. Interaction of the ultrasonic beamwith the sample results in a spatial variation of the plate boundaryconditions, changing the degree of excitation or frequency of generatedLamb modes at the defect locations. In the upper portion of each ofFIGS. 5,6 are three simulated delaminations 51a-c,61a-c each 25 mm indiameter, located farthest to nearest the isonified (upper) surface50a,60a of the sample as FIGS. 5,6 are viewed from left to right. Thenext two rows contain successively smaller delaminations 52a-c,62a-c and53a-c,63a-c. Next below are two rows of resin rich areas 54,55,64,65followed by a rows 56,66 of simulated porosity. It is evident from FIG.5 that some defects were detected without frequency modulation, but manydefects are either only weakly observed or not detected at all. Inaddition, it is clear that other larger scale variations, such asindicated at 57a-d, resulting from thickness variations are registered,which substantially obscure defects of interest because they shiftslightly the condition for excitation of the leaky Lamb waves.

In contrast, C-scan 60 of FIG. 6, wherein the same sample was examinedusing tone burst excitation and frequency modulation of the invention,clearly shows all nine delaminations 61a-c,62a-c,63a-c, and the resinrich regions 64a,b,65a,b toward the right side of the scan. Themicroporosity 66 near the bottom of C-scan 60 is observable when closeto insonified surface 60a, but is lesser defined in the sample center orremote of surface 60a. The heavy vertical striations are ply drops orgaps, which are obscured in scan 50 but are prominent in scan 60. Theless pronounced vertical features may be attributed to unevenlydispersed tow bundles in the sample.

The invention provides an improved nondestructive evaluation methodutilizing the generation and detection of leaky Lamb waves in compositelaminates utilizing tone burst excitation and frequency modulation ofincident ultrasonic energy in the laminate. It is understood thatcertain modifications to the invention as described may be made, asmight occur to one with skill in the field of the invention, within thescope of the appended claims. All embodiments contemplated herein whichachieve the objects of the invention have therefore not been shown incomplete detail. Other embodiments may be developed without departingfrom the spirit of the invention or from the scope of the appendedclaims.

I claim:
 1. A system for detecting defects in composite laminatematerial, comprising:(a) first transducer means for directing anultrasonic beam of preselected frequency along a transmission axis ontoa surface of a laminate material at preselected angle to generate Lambwaves in said material and leaky Lamb waves reflected from said surfaceof said material; (b) means for exciting said first transducer meanswith a radio frequency tone burst having preselected burst length andpreselected on-off duty factor; (c) second transducer means forreceiving said leaky Lamb waves reflected from said material along areflection axis and for providing an output signal corresponding to theamplitude of the reflected waves received by said second transducermeans; (d) means for recovering radio frequency signal amplitudeinformation from said output resulting from the tone burst excitation ofsaid first transducer means; (e) said first transducer means and saidsecond transducer means each disposed in preselected spaced relationshipto said surface of said material and to each other such that saidtransmission axis and said reflection axis intersect within saidmaterial a preselected distance below said surface; and (f) fluid meansfor sonically coupling said material with said first transducer meansand said second transducer means.
 2. The system of claim 1 furthercomprising means for selectively positioning said first transducer meansand said second transducer means along said surface of said material insaid spaced relationship.
 3. The system of claim 1 wherein said fluidmeans for sonic coupling of said material and said first transducermeans and said second transducer means comprises water.
 4. The system ofclaim 1 wherein said preselected frequency is in the range of from about0.5 to about 15 MHz.
 5. The system of claim 4 wherein said preselectedangle is in the range of from about 8° to about 75° relative to a normalto said surface of material.
 6. The system of claim 1 wherein saidpreselected burst length is from about 10 to about 200 microseconds. 7.The system of claim 1 wherein said preselected on-off duty factor isfrom about 0.001 to about 0.20.
 8. The system of claim 1 wherein saidmeans for recovering radio frequency amplitude information comprises agated integrating amplifier.
 9. A method for directing defects incomposite laminate material, comprising the steps of:(a) directing anultrasonic beam of preselected frequency along a selected transmissionaxis through a fluid sonic coupling bath onto a surface of a laminatematerial at preselected angle to generate Lamb waves in said materialand leaky Lamb waves reflected from said surface of said leaky material;(b) gating said preselected frequency to produce a radio frequency toneburst excitation directed along said selected transmission axis; (c)modulating said frequency of said beam over a preselected percentagerange of said frequency at preselected modulating rate; (d) receivingsaid leaky Lamb waves reflected from said surface of said material alonga reflection axis intersecting said transmission axis within saidmaterial at a preselected distance below said surface, and providing anoutput signal corresponding to the amplitude of the reflected waves; and(e) envelope detecting said output signal to remove radio frequencycontent thereof.
 10. The method of claim 9 further comprising the stepof(f) audio-frequency filtering said output signal to effect apreselected level of suppression of unwanted signal information.
 11. Themethod of claim 10 further including the step of selectively positioningsaid first transducer means and said second transducer means along saidsurface of said material for performing steps of (a) through (f) at aplurality of locations along said surface.
 12. The method of claim 9wherein said preselected frequency is in the range of from about 0.5 toabout 15 MHz.
 13. The method of claim 12 wherein said preselected angleis in the range of from about 8° to about 75° relative to a normal tosaid surface of said material.
 14. The method of claim 9 wherein saidfluid sonic coupling bath comprises water.